Today's engineers strive toward the goal of perfecting the efficiency of automotive engines by achieving lower emissions, better fuel efficiency, and improved performance. Government regulations require on-board diagnostics computers (On-Board Diagnostics or OBD) to further these goals. OBDs have been employed in motor vehicles since the late 1980s. The OBD system has been vastly improved to On-Board Diagnostics Level 2 ("OBD II"). OBD II not only monitors the partial failure of components but the deterioration of components as well.
With respect to the goal of lowering emissions, feedback control has been implemented with the engine/emission system to ensure that the optimum mixture of the gases is delivered to the catalytic converter. In general, an emissions system includes a three-way catalyst in the exhaust path to target particular components of the exhaust gases for the purpose of converting the targeted components to more environmentally friendly gases. For example, the three way catalysts convert HC, CO and NO, from the exhaust gas to H.sub.2 O, CO.sub.2 and N.sub.2.
Oxygen sensors have proven helpful in providing feedback control in emissions systems. In FIG. 1a, a typical oxygen sensor of the prior art and its conventional implementation in an emissions system are illustrated. The oxygen sensor 12 includes an oxygen permeable material and is generally mounted onto the exhaust system 14 near the exhaust manifold (not shown). The oxygen sensor 12 is used to maintain a stoichiometric air-fuel ratio by monitoring the oxygen content of the exhaust gas 16. The sensor 12 compares the oxygen level in the outside air 18 to the oxygen level in the exhaust gases 16. The sensor may further include a platinum tip 20 which is in direct contact with the exhaust gases 16. The platinum in the tip 20 equilibrates the gases and develops a voltage signal which is sent to a powertrain control module (not shown) for purposes of providing feedback to the air-fuel delivery system.
With reference to FIG. 1b, a prior art OBDII monitoring system 13 is illustrated. This prior art system monitors the oxygen storage ability of the catalyst. This indirect system and method of the prior art infers the deterioration in the hydrocarbon efficiency from changes in oxygen storage in the catalyst over time. As shown in FIG. 1b, a first oxygen sensor 22 is positioned at the upstream end 23 of a catalyst 24 and a second oxygen sensor 26 is positioned at the downstream end 28 of the catalyst. The first oxygen sensor 22 and the second oxygen sensor 26 measure the change in oxygen storage across the catalyst. The catalyst 24 may include oxygen storage material such as cerium. The first oxygen sensor 22 and the second oxygen sensor 26 collect data which track the oscillation between a rich condition and a lean condition of the exhaust gas. The collected data may be transmitted to a corresponding first signal processing means 28 and a second signal processing means 30 which are in communication with a means 32 for determining the switch ratio of the outlet sensor 26 to the inlet sensor 22. In determining the switch ratio, the system compares the rich-lean oscillation of the exhaust gas at the upstream side to the rich-lean oscillation at the downstream side of the catalyst.
However, as described above, this system and method do not provide a direct means of measuring oxidation efficiency given that the two sensors infer the oxidation efficiency based upon the catalyst oxygen storage capacity determined from a measurable quantity such as a voltage difference or a switch ratio. This indirect method assumes that the hydrocarbon efficiency of the system is adverse where the oxygen storage capacity of the catalyst is also adverse. However, the correlation between these two factors is approximately 0.6 to 0.7 at best. As known by those skilled in the art, the oxygen storage capacity may greatly vary despite very little change in the hydrocarbon efficiency.
Under a new emission-control system, a conditioning catalyst may be implemented for purposes of preventing "lean shift" in the control air-fuel ratio. "Lean shift" is a problem which occurs when the sensor incorrectly indicates that the hydrogen-laden exhaust gas is too rich and causes the engine control system to provide a leaner air fuel mixture. Conditioning catalysts help ensure accurate air-fuel control where very low emissions levels are mandated (e.g., California's Super Ultra Low Emission Vehicle (SULEV) standards). In this type of system, the exhaust gas stream is conditioned by a catalyst before the exhaust gas stream gets to the oxygen sensor by oxidizing the hydrogen and some of the hydrocarbons so that the sensor may more accurately measure the true air-fuel ratio of the exhaust. However, the catalyst in this system does not contain oxygen storage material given that the sensor must detect the oscillation between a rich condition and a lean condition at the downstream end of the conditioning catalyst to provide feedback control to the engine.
With negligible oxygen storage capacity in the catalyst, the conventional OBDII method of monitoring the deterioration of a catalyst (with dual sensors as described above) is no longer useful given that oscillation between a rich condition and a lean condition at the upstream side of the catalyst will be the same as the oscillation at the downstream side of the catalyst. Accordingly, a need has developed for a system and direct method that accurately diagnose the efficiency of a conditioning catalyst having negligible oxygen storage capacity.